What size is this driver?

Ever seen a driver where you can't tell the size from the specs? Perhaps they forgot to mention it.


Here is a general guide. You can tell from the "effective piston area" (SD) in the specs

3" driver - SD: 30 cm2
4" driver - SD: 57 cm2
5" driver - SD: 85 cm2
6.5" driver - SD: 135 cm2
8" driver - SD: 235 cm2
10" driver - SD: 350 cm2
12" driver - SD: 500 cm2
15" driver - SD: 800 cm2
18" driver - SD: 1180 cm2
21" driver - SD: 1550 cm2
24" driver - SD: 2200 cm2

Note: Pro driver with an accordion surround might have a higher SD for a given nominal size. The nominal size is usually based on the outer dimension.